DC power systems are gaining more popularity with increased DC loads (IT equipment, variable speed motor drives, LED lighting, etc.), renewable power generation (photovoltaic, wind, etc.), and distributed energy resources (batteries, fuel cells, microturbines, small gas generators). This is especially true for data centers, residential/commercial buildings, shipboard power systems, and high voltage DC transmission systems (HVDC), for example as described in “Low-Voltage DC Distribution System for Commercial Power Systems With Sensitive Electronic Loads,” D. Salomonsson and A. Sannino, IEEE Trans. Power Delivery, Vol. 22, No. 3, pp. 1620-1626, July 2007. DC power architectures improve energy efficiency by eliminating several conversion stages between AC/DC required in AC power systems. However, one major technical challenge for DC systems is to provide circuit breaker (CB) protection in the event of short circuit faults.
Electromechanical DC circuit breaker products are available from companies like ABB, Eaton, and Schneider Electric, but suffer from slow action, typically with a response time of 30-100 milliseconds, and limited lifetime due to arcing. DC solid-state circuit breaker (SSCB) solutions were also developed to provide a much faster response time, typically with a response time of tens to hundreds of microseconds, and a much longer lifetime. Power semiconductor switching devices such as silicon IGBTs or thyristors, and more recently silicon carbide (SiC) JEFTs (Junction Effect Transistors) or SITs (Static Induction Transistors) are often employed in these solutions. For further reference see: “Solid-state circuit breakers for Medium Voltage DC power,” Kempkes, M., Roth, I., Gaudreau, M., IEEE Electric Ship Technologies Symposium (ESTS), 2011, Page(s) 254-257; U.S. Patent Application Publication 2014/0029152 A1; U.S. Patent Application Publication 2013/0154391; and “SiC-SIT Circuit Breakers With Controllable Interruption Voltage for 400-V DC Distribution Systems,” Y. Sato et al., IEEE Tran. Power Electronics, Vol. 29, No. 5, May 2014, Page(s):2597-2605.
One challenge of these “smart” SSCBs is that they typically rely on complex and expensive over-current sensing circuitry, signal processing and data communication functions, and one or more external power supplies, which may or may not be available during the same short circuit fault event. A simple, stand-alone, two-terminal SSCB as reliable as the conventional electromechanical AC circuit breaker without requiring external power supply is highly desirable.
An objective of this invention is to develop a family of unidirectional and bidirectional SSCBs that do not require an external power supply to provide current interruption protection during the event of a short circuit fault in a DC power system. All known solutions rely on sensing over-current through a switch during a short circuit fault event and require one or more separate power supply to power up the control electronics of the SSBC. A preferred embodiment of this invention does not sense an over-current through a switch but rather a voltage across the switch to detect a short circuit fault. The sudden increase in the voltage across the switch, “desaturation,” provides the power to turn and hold off the switch until the short circuit condition is removed. In a preferred embodiment, the stand-alone SSCB of this invention does not require or draw any power in the conduction state for normal operation. The stand-alone SSCB uses a very small amount of leakage power to cut off a fault current when a short circuit condition is present. The power is drawn from the high voltage built across the switch using a DC-DC converter. For example, one or more isolated low wattage flyback DC-DC converters can be used for this purpose. These DC-DC converters must offer very fast dynamic response but relatively relaxed efficiency and voltage regulation requirements. This invention is simpler and more reliable than the prior art. In addition, the embodiments of the invention require a smaller die size of the core semiconductor switches than the prior art, resulting in considerable cost reduction.